Record-Holding Bridge

For more than 50 years, a pontoon-supported bridge crossing Lake Washington was recognized as the longest floating bridge in the world. In 2016, that claim passed to a newly constructed floating bridge measuring 130 feet longer than the old bridge, but located just a few feet away from the former record holder.

On April 2, 2016, a public grand opening celebration marked the opening of the $1.5 billion Evergreen Point Floating Bridge along State Route 520, which crosses Lake Washington in Medina, Washington, located to the east of downtown Seattle. At the grand opening, the bridge was certified as the longest floating bridge in the world by representatives from Guinness Book of World Records.

The previous floating bridge had exceeded its design life and was showing its age. The bridge pontoons had become vulnerable to windstorms, and its support columns were vulnerable to earthquakes. Constructed adjacent to the original floating bridge, the wider, sturdier bridge features 7,710 feet of floating span.

The bridge is owned and maintained by the Washington State Department of Transportation (WSDOT).

Serving as a member of the general engineering consultant team led by HDR, Inc., WSP USA provided program and project management, tolling and financial analysis, environmental documentation, preliminary engineering, design-build procurement, design and construction oversight and quality management. One of WSP’s key roles included providing revenue forecasting and financial planning for the $4.6 billion program and managing the preparation of revenue forecasts and toll scenario development and evaluation. The firm also assisted the owner in obtaining a $300 million Transportation Infrastructure Finance and Innovation Act (TIFIA) loan.

The State Route 520 program had multiple construction packages, the main one being the Floating Bridge and Landings Project, which had an overall approximate value of $1.5 billion. WSP was integral to the team that prepared preliminary engineering and procurement documents in a very aggressive schedule for this design-build project. The firm’s structure engineers also advised the owner to address constructability issues with the early pontoons and provided successful solutions.

Used by an average of 74,000 motorists every day, the new bridge provides two general-purpose lanes and one transit/high-occupancy vehicle (HOV) lane in each direction. It also features wider, safer shoulders that will allow vehicles to pull over in the case of a breakdown. It supports non-motorized traffic with a 14-foot-wide bicycle and pedestrian path on the north side of the bridge, and has the ability to accommodate future light rail if the region chooses to fund it in the future.

Key Numbers

Daily Vehicle Use

74,000

Length Longer Than Old Bridge

130 feet

New Floating Bridge Length

7,710 feet

Pontoons

77

Wind Speed Resistance

89 mph

Why Choose a Floating Bridge?

When project planning began, there was little doubt that a floating bridge was the most practical option. The middle of Lake Washington is 200 feet deep with poor soil conditions. Engineers determined that building any other type of bridge would have been difficult and more costly.

The new bridge also addressed the seismic vulnerability of the old bridge, which was built at a time when less consideration was given to potential earthquake damage. The original approaches on the bridge were built on hollow piers, which are not seismically stable. Now the piers are reinforced with much stronger support columns.

The new bridge is supported by 77 pontoons, including 21 that are 75 feet wide and 360 feet long, each measuring longer than a football field.

While the previous four-lane bridge was frequently closed when wind speeds reached 35-40 miles per hour, the new bridge is resistant to winds up to 89 miles per hour and should require closure only during extreme windstorms.

The new bridge also addresses storm water concerns. With the original bridge, unfiltered storm water ran straight into Lake Washington. The new bridge has an enclosed water filtration system, so the water is cleaned before it returns to the lake.”

Although most of the bridge was kept low to accommodate concerns from residents, a higher section allows for boat passage without halting traffic.

The new bridge improves transit reliability and travel times, and is the key feature in the seven-mile corridor series of improvements. The 74,000 vehicles that use the route every day can now enjoy a bridge that is reliable and up to current code.